This project seeks to provide the proof-of-concept for antiangiogenic immunotherapy targeting the premetastatic niche. We have found that complement C5a receptor (C5aR)-dependent angiogenesis begins in the lungs prior to the arrival of tumor cells and facilitates metastases; thus, this angiogenesis contributes to the premetastatic niche. These findings broaden our understanding of cancer-associated angiogenesis, since it is currently thought that angiogenesis begins in metastasis-targeted organs after metastases are present, facilitating the progression of dormant lesions to rapidly growing metastatic tumors. Leveraging our expertise in Listeria monocytogenes-based therapies and complement biology, we propose to use a combination of Listeria-based antiangiogenic vaccines and the inhibition of complement C5a receptor (C5aR) to stop premetastatic angiogenesis and metastases. This project is based on our recently published work demonstrating that inhibition of C5aR can prevent lung and liver metastases by enhancing antitumor CD8+ T-cell responses in these organs. The mechanisms to explain these beneficial effects include the complement- dependent reduction of lung and liver infiltration by myeloid-derived suppressor cells (MDSC), which are often blamed for the failure of various anticancer immunotherapies in the clinic. Therefore, we hypothesize that targeting MDSC through C5aR inhibition will synergize with and potentiate the actions of Listeria vaccines targeting premetastatic angiogenesis. Through these studies, we will identify the mechanisms regulating vascular alterations in the premetastatic niche that facilitate metastasis and design antiangiogenic therapy to target this niche. This strategy has the potential to prevent, delay, or reduce the metastases to vital organs that are responsible for 90% of cancer deaths. We will design an anticancer immunotherapy with minimal adverse effects, in contrast to the currently used toxic chemotherapy regimens. We anticipate that this active antiangiogenic immunization will induce a long-lasting immune response to the pathological vasculature, eliminating or reducing the need for frequent or prolonged administration of C5aR inhibitor and vaccines. Toward the goals of this application, we propose two well-integrated aims that will inform each other: (1) to identify the mechanisms regulating vascular alterations in the premetastatic niche, and (2) to compare the efficacy of the combined immunotherapy involving C5aR inhibition and Listeria antiangiogenic vaccines to monotherapy involving only one of these interventions and to the currently approved antiangiogenic therapies; and to identify the mechanisms of the synergistic action of C5aR inhibition and antiangiogenic vaccines. These aims will be achieved through in vivo studies in clinically relevant models of cancer.